UK children are losing contact with nature at a "dramatic" rate, and their health and education are suffering, a National Trust report says.
Traffic, the lure of video screens and parental anxieties are conspiring to keep children indoors, it says.
Evidence suggests the problem is worse in the UK than other parts of Europe, and may help explain poor UK rankings in childhood satisfaction surveys.
The trust is launching a consultation on tackling "nature deficit disorder".
"This is about changing the way children grow up and see the world," said Stephen Moss, the author, naturalist and former BBC Springwatch producer who wrote the Natural Childhood report for the National Trust.
"The natural world doesn't come with an instruction leaflet, so it teaches you to use your creative imagination.
"When you build a den with your mates when you're nine years old, you learn teamwork - you disagree with each other, you have arguments, you resolve them, you work together again - it's like a team-building course, only you did it when you were nine."
The trust argues, as have other bodies in previous years, that the growing dissociation of children from the natural world and internment in the "cotton wool culture" of indoor parental guidance impairs their capacity to learn through experience.
It cites evidence showing that:
children learn more and behave better when lessons are conducted outdoors
symptoms of children diagnosed with ADHD improve when they are exposed to nature
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